Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Keeping pace with advances in digital data processing, magnetic recording media such as Hard Disk Drives (HDD) has also been evolving to maintain continued growth in their data storage capacity. Specifically, efforts have been to increase areal density to reduce or maintain their size even as storage capacity keeps pace with increasing requirement.
To enable high storage densities in magnetic recording, significant efforts have been put on storage techniques like Heat-Assisted Magnetic Recording (HAMR) and Bit-Patterned Media (BPM). Since these techniques call for radical changes in the conventional media, feasibility of ultra-high storage densities ˜10 Tb/in2 on conventional media have been explored simultaneously by using shingled writing and two-dimensional readback. The scheme also known as two-dimensional magnetic recording (TDMR) is an exciting new option for ultra-high storage densities. But, TDMR channels come with a price of 2-D Inter Symbol Interference (ISI) and noise. Therefore, signal processing becomes significantly difficult in comparison to that in traditional 1-D recording.
In known proposed techniques for TDMR, the two-dimensional readback signal goes through process of equalization which shapes the readback magnetic recording signal to a specified partial response (PR), followed by Maximum Likelihood (ML) detection. The technique is widely used in 1D Magnetic Recording and the linear equalization allows controlled ISI called Generalized Partial Response (GPR) target.
Matcha and Srinivasain their paper titled “Target design and low complexity signal detection for two-dimensional magnetic recording” (Published in IEEE Annual Summit and Conference of Asia-Pacific Signal and Information Processing Association (APSIPA), pp. 1-10, 2014) have proposed techniques to design separable and non-separable 2-D PR targets and equalizers under monic and unit energy constraints using the MMSE criterion over a 2-D ISI channel with additive white Gaussian noise (AWGN). In their paper titled “Generalized Partial Response Equalization and Data-Dependent Noise Predictive Signal Detection Over Media Models for TDMR” (Published in IEEE Trans. Magn., vol. 51, no. 10, 2015), they further extended these techniques to TDMR channels using a Voronoi-based granular media model. S. Navabi and B. V. K. Vijaya Kumar, in their paper titled “Two-Dimensional Generalized Partial Response Equalizer for Bit-Patterned Media” (Published in IEEE International Conference on Comm., 2007) describe a method for joint equalization with PR targets for bit-patterned media storage.
FIG. 1 illustrates a typical block diagram of non-adaptive Partial Response Maximum Likelihood (PRML) detection in case of TDMR. Here, the signal received from the read channel is equalized using a linear equalizer before the signal is detected using a ML detector. The linear equalizer reduces the extent of ISI and achieves a desired overall response called the partial response (PR). This reduces computational complexity of the ML detector with some compromise in the performance.
The PR design techniques typically deal with minimizing the mean-squared error (MMSE) with constraints on the target such as the unit energy and the monic constraints.
These techniques, however, fail to take care of the time-varying nature of SNR under dynamic conditions. Thus, the equalization process is non-adaptive which calls for a better solution that can tackle the issues arising out of the time-varying nature of magnetic recording channels. There is, therefore, a need in the art for a method and apparatus wherein the read channel is cognizant of channel conditions i.e. it takes into account the time-varying nature of the channel which can help to mitigate the effects of SNR variations along with ISI reduction/control.
All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.